Does anyone know how to add a big black letterbox frame around an image, to cut a long story short, it needs to fit into a 6x8 frame, but it's portrait crop. THe frame has already been made, and I took my image in landscape mode. My client didn't make this clear, or I may of not listened properly, :( THe frame shop owners said if i put a black frame around it, it will fit.

I know how I do it, but the explanation would make it sound much more complicated & time consuming than it is

If your frame is 6x8 & your image is 5x7, you can use a custom-cut matte board as a frame within a frame, which would be a 5x8 matte board with a 5x7 hole cut in the center. Or, you can print it 6x8 & leave a 1 inch white border all around.

In PS, after your image is ready, use Canvas Size to add a border all around your image, in effect printing the matte board.